The invention is concerned with wireless remote control systems. While the invention is not to be so restricted, it will be discussed in the environment of a wireless remote control system for a television receiver. Such systems normally include a transducer for converting control signals into corresponding electrical signals, decoding circuits for determining which functions has been selected, and utilization circuits for performing the selected function. Various forms of noise immunity circuits are also included to enable such systems to distinguish between intentionally generated control signals and spurious signals.
Ultrasonic remote control systems have for years been a popular control medium for television receivers. A typical system consists of a viewer actuated transmitter for producing ultrasonic control signals at discrete frequencies and a remote receiver in the television set for receiving and decoding the control signals. The transmitter may be either electronic, and include an oscillator, or mechanical and employ rods of selected length and material which, when mechanically struck, produce distinctive ultrasonic frequencies corresponding to the selected function. A transducer in the receiver converts received ultrasonic information into corresponding electrical information which is supplied to a series of resonant circuits, each responsive to a particular one of the ultrasonic control signal frequencies. The resonant circuits thus determine which control signal has been received and activated appropriate utilization circuits in the television receiver.
In the ultrasonic region, noise produced, for example, by the jingling of keys or dropping of coins on hard surfaces may cause false actuation of individual functions of the television receiver. Recognizing that control signals and noise are distinguishable on the basis of amplitude and duration, a common form of noise immunization used R-C networks for integrating the relatively long duration control signals. Noise or other short duration signals were generally incapable of actuating the utilization devices. Another form used noise information to reverse bias the remote amplifier, and in effect, preclude operation in the presence of noise.
The resonant circuits for determining the control signals generally incorporated inductors. In the natural development of the art, the vacuum tubes used in the remote amplifiers were supplanted by transistors. Recently major portions of the television receiver circuitry are being fabricated in integrated circuit form, which has attractive space, cost and reliability advantages. The discrete inductors and (to a more limited extent) capacitors in the tuned circuits of the prior art remote receivers do not facilitate the integrated circuit form.
U.S. Pat. No. 3,611,297 to Kramer describes one system, which may be suitable for fabrication in an integrated circuit employing digital techniques. A clock is coupled to counters which sample an incoming signal to determine its frequency. Similar to the above mentioned prior art systems, noise immunity is achieved by a detector and noise suppressor circuit which requires that a valid control signal have a constant amplitude for some fixed period of time. The output of the detector and noise suppressor is coupled through a pulse shaper to a clock controlled frequency counter.
The counter uses a multi-stage counting technique with each stage dividing down the number of pulses to be counted by the succeeding stages. It appears that the least sensitive stages are ignored, the intermediate stages correspond to individual control functions and the most significant counter stages correspond to the proper range for the received frequency. The control function is activated only when the last stages detect the proper range and the intermediate stages decode one of the selectable functions. It is not clear from the Kramer disclosure how the circuit for noise detection operates. Apparently, the noise circuitry relies on the assumption that noise can be distinguished from control signals because of the former's randomly varying amplitude. The specification indicates that a noise suppressor circuit feeds a signal to circuitry for disabling the functioning of the counters during noise conditions. Thus, rather than distinguish between signal and noise, the system will shut down during reception of any varying amplitude signal even though it be a valid control signal. Thus, movement of the transmitter may cause misinterpretation of a valid control signal, for example, by waving the transmitter while producing a control frequency. Further, in the Kramer system the integrator in the noise suppressor operates on whatever signals are received and is incapable of distinguishing separate signals. It decodes whatever frequency signal is received and assumes that that frequency was present during the entire integration period. Simultaneously producing two or more frequencies or even a single frequency and a noise burst could confuse the system. The invention resolves these and other problems of the prior art by verifying a signal before decoding to activate the corresponding control function.